Colloid-facilitated transport of lead in natural discrete fractures

Colloid-facilitated transport of lead (Pb) was explored in a natural chalk fracture with an average equivalent hydraulic aperture of 139 μm. Tracer solution was prepared by adding montmorillonite (100 mg L ^-1) and/or humic acid (HA) (10 mg L ^-1) to modified artificial rainwater containing dissolved Pb (21.4 mg Pb L ^-1), naturally precipitated PbCO ₃ particles (16.4 mg Pb L ^-1) and LiBr (39.0 mg L ^-1). We found that Pb is only mobile when associated with colloids. PbCO ₃ particles were not mobile in the fracture. The addition of HA to the montmorillonite suspension increased the suspension's mobility and therefore promoted the colloid-facilitated transport of Pb. The increases in pH and sodium absorption ratio induced by the chalk-tracer solution interactions appeared to increase the dispersion and mobilization of colloids entering the fracture. The dominant colloid-facilitated transport of Pb reported in this study has significant implications for risk assessment of Pb mobility in fractured rocks.